Thus far, the following recording mediums are known for obtaining recording with visible-range absorption directly by heat. A heat-sensitive recording medium used with a facsimile machine or the like is made by applying a painting solution to an opaque support member such as paper which is obtained by dispersing and mixing a colorless or light-colored electron donor substance (leuco dye) and an electron acceptor substance (developer) in an aqueous solution of a water-soluble binder and atomizing the mixed particles. Other transparent recording media employing a nitrogen- or sulfur-containing onium compound as both a developer and thermal activator are prepared by dissolving a dye and developer in an organic solvent and applying the obtained solution to a transparent support member, which are disclosed in Japanese Patent Laid-Open (ko-kai) Nos. 294088/1989 and 45087/1988. These are materials for recording an image by a direct heating method in which a recording medium is directly heated with a thermal head kept in contact with it.
On the other hand, members and methods for recording an image by irradiating near-infrared rays rather than by direct heating, are disclosed in Japanese Patent Laid-Open (ko-kai) Nos. 94494/1983, 2880/1984 and 42994/1984. Japanese Patent Laid-Open (ko-kai) No. 94494/1983 discloses an optical recording medium prepared by applying a coating liquid comprising not only a dye and a developer but also a near-infrared absorbing agent dispersed in a water-soluble binder to a support member. Japanese Patent Laid-Open (ko-kai) Nos. 2880/1984, 42994/1983 disclose optical recording media prepared by individually laminating a dye, a developer, and a near-infrared absorbing agent on a substrate by vacuum evaporation. Also, Japanese Patent Laid-Open (ko-kai) No. 209594/1983 discloses a transparent recording medium wherein so-called "background fogging" is prevented by the provision of an isolating layer prepared by coating a solution of a near-infrared absorbing agent in the interface between the coating layer prepared by applying a solution of a leuco dye dissolved in an organic solvent to a substrate and that of a developer solution provided thereabove.
Japanese Patent Laid-Open (ko-kai) No. 177950/1993 discloses a transparent recording medium wherein the relevant "background fogging" is completely prevented. In Japanese Patent Laid-Open (ko-kai) No. 177950/1993, a complete prevention of "background fogging" is implemented by the provision of a transparent recording layer formed by coating a solution, comprising a developer expressed in the general formula (5) mentioned above for protecting at least one of functionally developable phenol-type hydroxide and a metal salt of organic acid or a leuco dye to develop a color in reaction with said developer dissolved in an organic solvent, onto a transparent substrate. In Japanese Patent Laid-Open (ko-kai) No. 177950/1993, the transparent recording medium is employed as an OHP (overhead projector) film, slide, copying draft, or a photo mask for preparing the circuit pattern of a resin circuit board or an integrated circuit board. Near-infrared absorbing materials include immonium compounds such as IRG002 (trade name) and IRG022 (trade name) available from Nippon Kayaku K.K., dithiol nickel complexes, cyanine dyes such as 1,1,5,5-tetraxis (p-dimethylaminophenyl)-3-methoxy-1,4-pentadiene toluene, squalerium dyes, naphthoquinone dyes, phthalocyanine compounds, and naphthalocyanine compounds.
Furthermore, as an application of this transparent recording medium, a masking material for print platemaking can be mentioned. With computerization of the printing process, various image forming methods interchangeable with conventional silver salt photography method are examined in the platemaking step and also as proofs for color proof-correction. In contrast to the silver salt photography method dealing chiefly with analog images, the laser recording method, the electronic photography method, the thermal transcription method, and the like can also form an image by using a digital signal and can provide a high quality printing block material and color correction proof at high speed. As a digital recording device, semiconductor lasers are most often employed from the standpoint of small size, durability, direct convertibility, low price and the like. As near-infrared sensitizers responsive to near-infrared rays irradiated from a semiconductor laser, employed are near-infrared absorbing materials such as phthalocyanine coloring substances described in Japanese Patent Publication (ko-koku) No. 64064/1991, heptamethine cyanine coloring substances described in Japanese Patent Publication (ko-koku) No. 28143/1990, naphthoindolenine-type cyanine coloring substances described in Japanese Patent Laid-Open (ko-kai) No. 273959/1994) and the like. Also, with respect of image quality, silver salt photography still occupies the highest position as the image forming method, but needs treating liquids for a developer, bleaching/fixing solution and the like. Since the maintenance and control of treating liquid quality is troublesome, the treatment time is long, the disposal of development treating liquids is serious and so on, materials replacing silver salt photograph are being called for.
Generally, near-infrared absorbing materials are often employed for optical recording media such as optical disks. Optical disks give high-density recordings by irradiating near-infrared laser rays to a recording layer provided on a circular support member. A near-infrared absorbing material contained in the recording layer efficiently converts near-infrared laser rays into heat and effects a thermodynamic deformation such as decomposition, vaporization and dissolution. Playback of the recorded information is carried out by reading the difference in reflectance between the generated portion of deformation by laser irradiation and the not-generated portion of deformation. Here, the performance required for near-infrared absorbing material is to have the maximum wavelength of absorption near the wave length of laser rays to be used for recording, exhibit a high extinction coefficient at this wave length, and have a high efficiency in converting light into heat. As examples of near-infrared absorbing materials employed for transparent recording mediums, there can be mentioned phthalocyanine-type coloring substances described in Japanese Patent Laid-Open (ko-kai) No. 97033/1980, phenalen-type coloring substances described in Japanese Patent Laid-Open (ko-kai) No. 83344/1983, naphthoquinone-type coloring substance described in Japanese Patent Laid-Open (ko-kai) No. 224793/1983, and indoaniline metal complexes described in Japanese Patent Laid-Open (ko-kai) Nos. 227569/1988, 254772/1989, 568/1990, 667/1990. Furthermore, attempts were made to efficiently absorb laser rays by modifying the structure of these indoaniline metal complexes in metal-contained compounds described in Japanese Patent Laid-Open (ko-kai) No. 73865/1990 and metal-contained pyridophenothiazone-type compounds described in Japanese Patent Laid-Open (ko-kai) No. 76884/1990, but neither a high recording density nor a high contrast between the recorded portion and the unrecorded portion was obtained, so that no basic solution has been achieved.
In forming an image on a transparent recording medium by using near-infrared laser rays as a light source, it is required that a near-infrared absorbing material contained in the transparent recording medium efficiently absorb and convert into thermal energy the energy of near-infrared laser rays having a predetermined wavelength used for an image recording into thermal energy. Using a transparent recording medium as the masking material for print platemaking to be employed in the printing step requires both a high recording density and a high contrast between the portion recorded by laser rays and the unrecorded portion. Japanese Patent Laid-Open (ko-kai) No. 227569/1988 discloses a single layer of indocyanine-type coloring substance provided on a substrate, but this indocyanine-type coloring substance has an absorption in the visible range and presents a problem in that a film of the single layer made by using this coloring substance develops a green color to form a recording medium lacking in contrast. Furthermore, cyanine-type coloring substances are generally subject to discoloration and fading by sunlight and have problems in light resistance and the like.
In addition, in applications to optical recording media, a single layer of phthalocyanine-type coloring substance provided on a substrate, described in Japanese Patent Laid-Open (ko-kai) No. 97033/1980, has problems of low sensitivity, a high decomposition point and difficult vapor deposition, and further, problems of low solubility in organic solvents and unavailability to coating by application. With the phenalene-type coloring substance described in Japanese Patent Laid-Open (ko-kai) No. 83344/1983 and the naphthoquinone-type coloring substance described in Japanese Patent Laid-Open (ko-kai) No. 224793/1983, because the efficiency of converting light into heat is low despite the easiness of vapor deposition, the thermal deformation in the portion irradiated by laser rays is insufficient and a contrast in reflectance between the recorded portion and the unrecorded portion decreases in the playback of information and the playback of information becomes difficult. Because of being generally inferior in stability for laser rays employed during playback, organic coloring substances have another problem that the conversion efficiency declines when additional-recording by laser rays, thereby disabling information to be recorded.
With due consideration for the present circumstances, it is one object of the present invention to provide a transparent recording medium, high in recording density, good in contrast and excellent in longevity such as light resistance, by using new indoaniline metal complexes and their compounds, high in near-infrared absorption, highly efficient in converting light into heat, low in visible absorption, and further, highly soluble in an organic solvent, capable of coating by application, and excellent in longevity such as light resistance.